1. Field of the Invention
This invention relates to a power steering mechanism which is adapted to control the required steering force by variably controlling the preload on rack teeth formed on a piston of a power cylinder and a sector gear formed on a sector shaft and meshed with the rack teeth.
2. Description of the Prior Art
Generally, a ball screw type steering mechanism includes a screw shaft, a ball nut mounted on the screw shaft through balls, and a sector shaft with a sector gear meshed with rack teeth formed on the ball nut. The conventional power steering mechanism of this sort has a function of responding to the vehicle speed, displacing the sector shaft in the axial direction by operation of a cylinder according to the vehicle speed thereby to control the preload on the rack teeth and sector gear in response to the vehicle speed. Namely, the preload is reduced to provide light steering in low speed operations of the vehicle and the preload is increased to provide heavy steering in high speed operations.
Accordingly, the conventional powering steering mechanisms have a drawback that the steering wheel becomes heavier in operation in high speed running of a vehicle.